1. Field of the Invention
The present invention relates to a valve device for gas drainage used for a metal mold for die-casting, and relates also to a metal mold for die-casting.
2. Description of the Related Art
In a die-casting method, molten metal is injected at high speed into a mold cavity. Therefore, the molten metal may suck up gases such as air inside the cavity. To prevent the sucking up of such gases, there is known a vacuum die-casting method in which the gases inside the cavity are sucked out with a vacuum pump before injecting the molten metal such that the cavity is set to a decompressed state.
In a vacuum die-casting machine employing the vacuum die-casting method, a valve device for selectively communicating and blocking a gas drainage path is installed in the middle of the gas drainage path connecting a vacuum pump installed outside the metal mold and the mold cavity. That is, when a valve element of the valve device is opened, the gas drainage path is communicated, the gases inside the cavity are removed by the vacuum pump, the cavity is set in a decompressed state, and when the valve element of the valve device is closed, the gas drainage path is blocked.
Japanese Unexamined Patent Application Publication No. H05-15961 discusses a vacuum die-casting machine equipped with the above-described valve device. This vacuum die-casting machine is of horizontal type, and the valve device is installed from above in a housing dent formed on the upper end of a stationary mold. The valve device is equipped with a cylinder so that the valve element is driven to open and close. A piston is housed in the cylinder, and this piston is coupled to the valve element via a valve stem. The cylinder is partitioned into a front chamber and rear chamber by the piston. An electromagnetic selector valve is provided between a compressor for supplying compressed air and the cylinder, and when the electromagnetic selector valve switches a supply route of the compressed air, the compressed air is supplied either to the front chamber or to the rear chamber of the cylinder. When the compressed air is fed into the front chamber, the valve element moves backward and the gas drainage path is blocked, and when the compressed air is fed to the rear chamber, the valve element moves forward and the gas drainage path is communicated. However, this valve device is installed on the upper end of the stationary mold, and is far away from the cavity. Therefore, a long gas drainage path is formed between the cavity and valve device. If the gas drainage path from the cavity to the valve device is long, then the amount of molten metal flowing in the gas drainage path becomes large, and the amount of molten metal consumed in one shot increases. Further, the efficiency of processing the molten metal that has solidified in the gas drainage path also deteriorates.